To study the human prion diseases, transgenic (Tg) mice expressing chimeric prion proteins (PrP) comprised of mouse and human PrP sequences have been produced. By mutating the residues individually that differ between human and mouse PrP, we have produced Tg mice that are progressively more susceptible to human prions as evidenced by shorter and shorter incubation times. We propose to develop Tg mice even more sensitive human prions and use these as well as existing Tg mice for the detection of human prions. The bioassay data will be compared to measurements of both the protease-sensitive (s) PrPSc as well as protease-resistant (r) PrPSc. We shall also study the tissue distribution of prion infectivity as well as sPrPSc and rPrPSc in both people and animal models. Our studies may elucidate some of the factors that feature in the transmission of prions between humans and animals. Currently, the Tg mice that are most susceptible to human prions are those expressing a chimeric Mo/Hu PrP, which differs from MoPrP at just six residues. These Tg mice are susceptible to prions from patients who died of sporadic (s) Creutzfeldt- Jakob disease (CJD), in just 80 days. We plan to study systematically the effect of each of these six residues on susceptibility to prion infection, and determine how the expression level of the transgene impacts the incubation time to disease onset after inoculation. We shall also use guinea pigs as models for human prion diseases, having recently demonstrated that they uniquely reproduce the pathological features of variant (v) CJD. The large brain and blood volumes of the guinea pig makes them useful for CNS imaging studies as well as investigations of prions in blood and lymphoid tissues. Several studies suggest that the distribution of prions in brain and lymphoid tissues are quite different in sCJD versus vCJD patients. Whether guinea pigs will reproduce these differences in prion distribution remains to be established. While only four cases of vCJD occurred in the UK last year, the possibility that vCJD prions have contaminated the human blood supply cannot be ignored. Besides measuring prion titers by bioassays in Tg mice, we plan to measure the levels of both sPrPSc and rPrPSc. Recent advances in methods for measuring particularly sprpsc usjng tne amyloid seeding assay (ASA) are encouraging.